IDENTIFICATION FROM A PHAGE DISPLAY LIBRARY OF PEPTIDES THAT BIND TO TOXIC SHOCK SYNDROME TOXIN-1 AND THAT INHIBIT ITS BINDING TO MAJOR HISTOCOMPATIBILITY COMPLEX (MHC) CLASS-II MOLECULES

Phage display technique is a powerful tool with which to identify nove l binding sequences for antibody and receptor targets. Few studies, ho wever, have used this technology to select affinity peptides for ligan d molecules. Here, we screened a peptide phage library for binding to toxic shock syndrome toxin 1 (TSST-1) to examine whether peptide ligan ds for TSST-1 which mimic the structure of major histocompatibility co mplex (MHC) class II receptors could be identified. After three cycles of biopanning, four potent sequences reactive with TSST-1 were isolat ed (designated phages 2, 3, 8, and 11). Selected phage were found to r eact specifically with TSST-1 but not with other staphylococcal exotox ins. A synthetic peptide (pep3) corresponding to the most frequently i dentified sequence (phage3) was shown to inhibit binding of all four i solated phage to TSST-1, suggesting that they bind to a common site on TSST-1. Furthermore, pep3 was shown to compete with MHC class II mole cules for binding to TSST-1 in a concentration-dependent manner. Compa rison of their sequences with MHC class II molecules revealed that pha ge8 shared sequence homology with two regions of the beta chain of MHC class II molecules: amino acids 57-62, containing a residue (Tyr-60) involved in TSST-1 binding as suggested by X-ray crystallographic data of TSST-1-MHC class II complex; and amino acids 188-193, a region not previously known as a contact domain. These results suggest that the selected sequences recognized the MHC class II binding site on TSST-1. Thus, affinity selection for peptides binding to ligand molecules (e. g., TSST-1) rather than their cognate receptors (e.g., MHC class II) f rom a random phage display library represents a useful approach to und erstanding receptor-ligand interactions.